1. Field of the Invention
This invention relates to a composition for a pressure-sensitive adhesive (hereinafter, referred to as pressure-sensitive adhesive composition) and a pressure sensitive adhesive sheet using the pressure-sensitive adhesive composition, and in particular relates to a pressure-sensitive adhesive composition which is suitably used for sticking optical films such as light-polarizing sheets and phase difference sheets and the like which are to be stuck onto optical parts such as a liquid crystal cell of a liquid crystal display device or for sticking shading films onto window glass, and a pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition.
2. Description of the Background Art
In optical parts such as a liquid crystal cell of a liquid crystal display device, a light-polarizing sheet is stuck thereto. In general, such a light-polarizing sheet has a three-layer structure including a polarizing element which is formed of a PVA(polyvinylalcohol)-based material and protective films which are attached to the opposite surfaces of the light-polarizing element and formed of TAC(triacetylcellulose)-based material, in which a release liner is attached onto at least one of the protective films by means of a pressure-sensitive adhesive layer.
When such a light-polarizing sheet is attached to the liquid crystal display device, it is stuck onto the liquid crystal cell through the pressure-sensitive adhesive layer after the release liner has been peeled off.
Such a light-polarizing sheet has poor dimensional stability due to its material properties. In particular, the light-polarizing sheet is subject to large dimensional changes due to expansion and contraction when it is put under heat or wet heat condition, with this result curl or the like being likely to be produced.
As for the pressure-sensitive adhesive (that is the pressure-sensitive adhesive composition), there is used a pressure-sensitive adhesive composition which is formed by adding a coupling agent of silane which contains epoxy-silane or the like as its main component and has one alkoxysilyl group in its molecular into a two-liquid curable type hardening resin composition which has high adhesive strength and high shearing modulus.
The use of such a silane coupling agent increases the cohesive strength of the pressure-sensitive adhesive. With this result, the pressure-sensitive adhesive layer formed of such a pressure-sensitive adhesive can not sufficiently absorb or relieve the stress which is resulted from the dimensional changes in the light-polarizing sheet caused under the heat or wet heat condition.
When such unfavorable situation would occur, the distribution of the residual stress exerted in the light-polarizing sheet becomes ununiform so that stress concentration occurs in particular at the peripheral portion thereof. As a result, displacement of the light-polarizing sheet occurs, which results in a problem in that a white appearance is likely to be produced in a TN (TFT) liquid crystal cell and a color unevenness is likely to be caused in a STN liquid crystal cell. Further, there is another problem in that lifting and peeling off of the light-polarizing sheet from the liquid crystal cell are also likely to be caused.
Further, under the wet heat condition, such lifting and peeling off of the light-polarizing sheet from the liquid crystal cell is more likely to be caused since water enters into the boundary face between the pressure-sensitive adhesive layer and the glass substrate. Therefore, it is required to prevent occurrence of such defects beforehand.